1. Field of the Invention
The invention relates generally to Viterbi trellis coding methods and apparatus, and more particularly to Viterbi trellis coding methods and apparatus for partial-response maximum-likelihood (PRML) data channels in a direct access storage device (DASD).
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks. Partial-response signaling with maximum-likelihood sequence detection techniques are known for digital data communication and recording applications. Achievement of high data density and high data rates has resulted in the use of a PRML channel for writing and reading digital data on the disks.
U.S. Pat. No. 4,786,890 discloses a class-IV PRML channel using a run-length limited (RLL) code. The disclosed class-IV partial response channel polynomial equals (1-D.sup.2), where D is a delay operator and D.sup.2 is a delay of 2 bit times and the channel response output waveform is described by taking the input waveform and subtracting from it the same waveform delayed by a 2 bit interval. A (0,k=3/k1=5) PRML modulation code is utilized to encode 8 bit binary data into codewords comprised of 9 bit code sequences, where the maximum number k of consecutive zeroes allowed within a code sequence is 3 and the maximum number k1 of consecutive zeroes in the all-even or all-odd sequences is 5.
Trellis coding techniques are used to provide a coding gain required in noisy or otherwise degraded channels. U.S. Pat. Nos. 4,888,775 and 4,888,779 describe trellis codes for PRML channels which provide significantly improved coding gains for transmission of digital data over PRML channels.
There is a need for improved Viterbi trellis code detectors that provide coding gains and bit rates required for transmission of digital data over PRML channels while reducing the number of calculations involved in the Viterbi algorithm, and reducing and simplifying the required digital hardware.